The present invention relates to a liquid crystal display device used for a personal computer, a work station or the like, and in particular to a technique useful for a video signal line driver circuit (a drain driver) in a liquid crystal display device capable of a multi-gray scale display.
Active matrix type liquid crystal display devices which have an active element (a thin film transistor, for example) for each pixel and switch the active elements are widely used as display devices for notebook personal computers and the like. In the active matrix type liquid crystal display device, each pixel electrode is supplied with a video signal voltage (a gray-scale voltage) via an active element, no cross talk between pixels occur, and therefore a multi-gray scale display is produced without the need for a special driving scheme for prevention of the cross talk, unlike a liquid crystal display device of the so-called simple matrix type.
As one of the active matrix type liquid crystal display devices, there is known a TFT (Thin Film Transistor) type liquid crystal display module having a TFT type liquid crystal display panel (a TFT-LCD), drain drivers disposed at the top side of the liquid crystal display panel and gate drivers disposed at the lateral side of the liquid crystal display panel.
The TFT type liquid crystal display module includes, in its drain drivers, a gray-scale voltage generating circuit for generating a plurality of gray-scale voltages, decoders for selecting a gray-scale voltage in accordance with a display data from among the plurality of gray-scale voltages generated by the gray-scale voltage generating circuit, amplifiers for amplifying the gray-scale voltage selected by the decoders so as to output a video signal voltage in accordance with the display data to a corresponding one of drain signal lines, and a bias circuit for controlling a current of a constant current source in the amplifiers. Such a technique is disclosed in Japanese Patent Application No. Hei 11-47885 (filed on Feb. 25, 1999, but not laid-open on the filing date of the present application), for example.
The gray-scale voltage generating circuit in the drain driver includes a voltage-dividing resistor circuit for dividing voltages between a plurality of gray-scale reference voltages supplied from a power supply circuit so as to generate a plurality of gray-scale voltages.
Each of the drain drivers is formed in one semiconductor integrated circuit (a semiconductor chip), and the voltage-dividing resistor circuit is formed of a tapped resistive element, a plurality of gray-scale voltage lines for outputting gray-scale voltages, an interlayer insulating film for insulating the gray-scale voltage lines from the tapped resistive element, and a plurality of connections for connecting the gray-scale voltage lines with the taps of the tapped resistive element via contact holes formed in the interlayer insulating film.
A resistance of the resistive element between two adjacent taps of the resistive element is determined by (a length of the resistive element between the two taps of the resistive element)/(a width W of the resistive element)xc3x97(a sheet resistance of the resistive element).
In conventional drain drivers, the above-mentioned connections for connecting the gray-scale voltage lines-with the taps of the tapped resistive element are disposed in a current path of a current flowing through the resistive element. In this case, the length L of the resistive element between the two adjacent taps varies with manufacturing variability of dimensions of the contact holes or the like, and consequently, a problem arises in that the resistance of the resistive element between two adjacent taps varies such that a gray-scale voltage generated in the voltage-dividing resistor circuit varies and the quality of a display image by the liquid crystal display panel is degraded.
The area of the contact holes had to be made small because the contact holes are disposed in a current path of a current flowing through the resistive element and therefore the area of the contact holes is limited. As a result, there has been a problem that the resistance at the connections for connecting the gray-scale voltage lines with the taps of the resistive element is increased and time delay is caused in transfer characteristics of gray-scale voltages from the voltage-dividing resistor circuit to a succeeding amplifier.
Recently, there are demands for a larger-sized display panel (a larger-sized TFT-LCD), higher resolution, a higher-quality display image and lower power consumption on the TFT active matrix type liquid crystal display device, and also there is a demand for reduction of power consumption on the liquid crystal display devices because necessity for their long-period operation powered by batteries is becoming greater as notebook personal computers spread.
In this case, for the purpose of improving the quality of a display image, the greater the voltage range of gray-scale voltages applied across the liquid crystal layer, that is, the voltage range of output voltages outputted from the drain drivers, the better for improvement of response speed of the liquid crystal and display contrast. In view of this, the power supply voltage VDD for the drain drivers is selected to be high.
In general, each of the amplifiers of the drain drivers comprises a high-voltage amplifier for amplifying positive-polarity gray-scale voltages and a low-voltage amplifier for amplifying negative-polarity gray-scale voltages. These high-voltage and low-voltage amplifiers are formed by differential amplifiers and current values of constant-current sources each for a respective one of the differential amplifiers are determined by one bias circuit. The bias circuit had to be formed by high-breakdown-voltage MOS transistors (hereinafter referred to merely as high-voltage MOS transistors) because the power supply voltage VDD for the drain drivers is high.
Generally, in the high-voltage MOS transistors, the thickness of the gate insulator oxide is made thick enough to ensure a high breakdown voltage (a high withstand voltage) and a region for relaxing electric fields is necessary, and consequently, variations in threshold voltages and the like of high-voltage MOS transistors are greater than those of low-breakdown-voltage MOS transistors (hereinafter referred to merely as low-voltage MOS transistors). As a result, current values of the currents supplied from the bias circuits to the constant-current sources of the differential amplifiers forming the amplifiers of the drain drivers vary from drain driver to drain driver, and in the liquid crystal panel incorporating about ten drain drivers there is a problem that there is a possibility that display brightness varies from drain driver to drain driver and the quality of a display of the liquid crystal display panel is degraded.
The present invention solves the above-mentioned problems with the prior art, and it is an object of the present invention to provide a technique for improving the quality of a display image of the liquid crystal display panel in the liquid crystal display device.
It is another object of the present invention to provide a technique for preventing occurrence of variations in respective gray-scale voltages generated by gray-scale voltage generating circuits in the liquid crystal display device.
It is yet another object of the present invention to provide a technique for making uniform the current values of the currents of the constant-current sources of the amplifiers of the drain drivers in each of the drain drivers by making it possible to use low-voltage MOS transistors in the bias circuits, in the liquid crystal display device.
The above objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.
The following explains a summary of representative configurations of the present invention.
To accomplish the above objects, in accordance with an embodiment of the present invention, there is provided a liquid crystal display device comprising a liquid crystal display element having a plurality of pixels arranged in a matrix and a plurality of video signal lines for applying a video signal voltage to each of the plurality of pixels in accordance with a display data, and a video signal line driver circuit for supplying the video signal voltages to the plurality of video signal lines, the video signal line driver circuit including a gray-scale voltage generating circuit provided with a voltage-dividing resistor circuit for dividing voltages between a plurality of gray-scale reference voltages supplied from an external power supply circuit so as to generate a plurality of gray-scale voltages, a plurality of selector circuits corresponding to the plurality of video signal lines for selecting one gray-scale voltage from among the plurality of gray-scale voltages in accordance with the display data, the voltage-dividing resistor circuit including a resistive element provided with a plurality of intermediate taps for dividing voltages between the plurality of gray-scale reference voltages so as to generate the plurality of gray-scale voltages, a plurality of gray-scale voltage lines corresponding to the plurality of gray-scale voltages, an interlayer insulating film for insulating the plurality of gray-scale lines from the resistive element, and a plurality of connections for electrically connecting each of the plurality of gray-scale voltage lines to a corresponding one of the plurality of intermediate taps through a hole formed in the interlayer insulating film, the plurality of connections being disposed at positions displaced from a current path of a current flowing in the resistive element.
To accomplish the above objects, in accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising a liquid crystal display element having a plurality of pixels arranged in a matrix and a plurality of video signal lines for applying a video signal voltage to each of the plurality of pixels in accordance with a display data, and a video signal line driver circuit for supplying the video signal voltages to the plurality of video signal lines, the video signal line driver circuit including a gray-scale voltage generating circuit provided with a voltage-dividing resistor circuit for dividing voltages between a plurality of gray-scale reference voltages supplied from an external power supply circuit so as to generate a plurality of gray-scale voltages, a plurality of selector circuits corresponding to the plurality of video signal lines for selecting one gray-scale voltage from among the plurality of gray-scale voltages in accordance with the display data, the voltage-dividing resistor circuit including a resistive element provided with a plurality of intermediate taps for dividing voltages between the plurality of gray-scale reference voltages so as to generate the plurality of gray-scale voltages, a plurality of gray-scale voltage lines corresponding to the plurality of gray-scale voltages, an interlayer insulating film for insulating the plurality of gray-scale lines from the resistive element, and a plurality of connections for electrically connecting each of the plurality of gray-scale voltage lines to a corresponding one of the plurality of intermediate taps through a hole formed in the interlayer insulating film, each of the plurality of intermediate taps forming a portion protruding in a direction of extension of the plurality of gray-scale voltage lines, from the resistive element and each of the plurality of connections being disposed on the protruding portion.
To accomplish the above objects, in accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising a liquid crystal display element having a plurality of pixels arranged in a matrix and a plurality of video signal lines for applying a video signal voltage to each of the plurality of pixels in accordance with a display data, and a video signal line driver circuit for supplying the video signal voltage to the plurality of video signal lines, the video signal line driver circuit including a plurality of amplifiers corresponding to the plurality of video signal lines, each of the plurality of amplifiers outputting the video signal voltage to a corresponding one of the plurality of video signal lines, and a bias circuit including a current mirror circuit for controlling a current in a constant-current source in each of the plurality of amplifiers, the current mirror circuit including, between a first power supply voltage line supplied with a first reference power supply voltage and a second power supply voltage line supplied with a second reference power supply voltage, a first transistor element of a first conductivity type and having a low breakdown voltage, a second transistor element of a second conductivity type and having a breakdown voltage higher than the low breakdown voltage, the second transistor element being connected in series with the first transistor element, and at least one third transistor element of the first conductivity type, the at least one third transistor element being connected between the first transistor element and the second transistor element and having a fixed bias voltage applied to a control electrode thereof, the fixed bias voltage being between the first and second reference power supply voltages.
To accomplish the above objects, in accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising a liquid crystal display element having a plurality of pixels arranged in a matrix and a plurality of video signal lines for applying a video signal voltage to each of the plurality of pixels in accordance with a display data, and a video signal line driver circuit for supplying the video signal voltage to the plurality of video signal lines, the video signal line driver circuit including a plurality of amplifiers corresponding to the plurality of video signal lines, each of the plurality of amplifiers outputting the video signal voltage to a corresponding one of the plurality of video signal lines, and a bias circuit including a current mirror circuit for controlling a current in a constant-current source in each of the plurality of amplifiers, the current mirror circuit including, between a first power supply voltage line supplied with a first reference power supply voltage and a second power supply voltage line supplied with a second reference power supply voltage, a first transistor element of a first conductivity type and having a low breakdown voltage, a second transistor element of a second conductivity type and having a breakdown voltage higher than the low breakdown voltage, the second transistor element being connected in series with the first transistor element, and at least one third transistor element of the first conductivity type, the at least one third transistor element being connected between the first transistor element and the second transistor element and having a control electrode thereof connected to a terminal thereof connected to the second transistor element.
To accomplish the above objects, in accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising a liquid crystal display element having a plurality of pixels arranged in a matrix and a plurality of video signal lines for applying a video signal voltage to each of the plurality of pixels in accordance with a display data, and a video signal line driver circuit for supplying the video signal voltage to the plurality of video signal lines, the video signal line driver circuit including a plurality of amplifiers corresponding to the plurality of video signal lines, each of the plurality of amplifiers outputting the video signal voltage to a corresponding one of the plurality of video signal lines, and a bias circuit for controlling a current in a constant-current source in each of the plurality of amplifiers, the bias circuit including (a) a first series combination comprising: a first transistor element of a first conductivity type and having a first low breakdown voltage; a second transistor element of a second conductivity type and having a breakdown voltage higher than the first low breakdown voltage, the second transistor element being connected in series with the first transistor element; and at least one third transistor element of the first conductivity type and having a breakdown voltage higher than the first low breakdown voltage, the at least one third transistor element being connected between the first transistor element and the second transistor element; a terminal of the second transistor element connected to the at least one third transistor element being connected to a control electrode of the second transistor element, and a control electrode of the first transistor element being supplied with a bias voltage; (b) a second series combination comprising: a fourth transistor element of the first conductivity type and having a second low breakdown voltage; a fifth transistor element of the second conductivity type and having a breakdown voltage higher than the second low breakdown voltage, the fifth transistor element being connected in series with the fourth transistor element; and at least one sixth transistor element of the first conductivity type and having a breakdown voltage higher than the second low breakdown voltage, the at least one sixth transistor element being connected between the fourth transistor element and the fifth transistors element; a control electrode of the fifth transistor element being connected to the control electrode of the second transistor element, a terminal of the fourth transistor element connected to the at least one sixth transistor element being connected to a control electrode of the fourth transistor element, and a control electrode of the fourth transistor element being configured so as to provide an output; wherein a parallel combination of the first series combination and the second series combination is connected between a first power supply voltage line supplied with a first reference power supply voltage and a second power supply voltage line supplied with a second reference power supply voltage, and a voltage intermediate between the first and second reference power supply voltages is applied to control electrodes of the at least one third transistor element and the at least one sixth transistor element.
To accomplish the above objects, in accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising a liquid crystal display element having a plurality of pixels arranged in a matrix and a plurality of video signal lines for applying a video signal voltage to each of the plurality of pixels in accordance with a display data, and a video signal line driver circuit for supplying the video signal voltage to the plurality of video signal lines, the video signal line driver circuit including a plurality of amplifiers corresponding to the plurality of video signal lines, each of the plurality of amplifiers outputting the video signal voltage to a corresponding one of the plurality of video signal lines, and a bias circuit for controlling a current in a constant-current source in each of the plurality of amplifiers, the bias circuit including (a) a first series combination comprising: a first transistor element of a first conductivity type and having a first low breakdown voltage; a second transistor element of a second conductivity type and having a breakdown voltage higher than the first low breakdown voltage, the second transistor element being connected in series with the first transistor element; and at least one third transistor element of the first conductivity type and having a breakdown voltage higher than the first low breakdown voltage, the at least one third transistor element being connected between the first transistor element and the second transistor element; a terminal of the second transistor element connected to the at least one third transistor element being connected to a control electrode of the second transistor element, and a control electrode of the first transistor element being supplied with a bias voltage; (b) a second series combination comprising: a fourth transistor element of the first conductivity type and having a second low breakdown voltage; a fifth transistor element of the second conductivity type and having a breakdown voltage higher than the second low breakdown voltage, the fifth transistor element being connected in series with the fourth transistor element; and at least one sixth transistor element of the first conductivity type and having a breakdown voltage higher than the second low breakdown voltage, the at least one sixth transistor element being connected between the fourth transistor element and the fifth transistor element; a control electrode of the fifth transistor element being connected to the control electrode of the second transistor element, a terminal of the fourth transistor element connected to the at least one sixth transistor element being connected to a control electrode of the fourth transistor element, and a control electrode of the fourth transistor element being configured so as to provide an output; wherein a parallel combination of the first series combination and the second series combination is connected between a first power supply voltage line supplied with a first reference power supply voltage and a second power supply voltage line supplied with a second reference power supply voltage, a control electrode of the at least one third transistor element is connected to a terminal of the at least one third transistor element connected to the second transistor element, and a control electrode of the at least one sixth transistor element is connected to a terminal of the at least one sixth transistor element connected to the fifth transistor element.